Process for hardening steel steet and strips by over-aging



United States Patent 3,117,897 PRGCESS F81 HARDENEJG STEEL SHEET AND S'IRlPS BY OVER-AGING Ernest Wynne Williams, Bishopston, Swansea, England,

assignor to The British Iron and Steel Research Association, London, England No Drawing. Filed Sept. 24, 1962, Ser. No. 225,831 Claims priority, application Great Britain Nov. 7, 1961 8 Claims. (Cl. 148-142) The present invention relates to a process for the annealing of cold worked steel sheet and strip and is more particularly concerned with a process for the production of plate having a hardness of at least 65 R30T units.

Steel sheet and strip is produced for the tinplate industry in a number of grades having different ranges of hardness and these grades of material are known in the British and American tinplate industries as Temper 1, Temper 2, etc. up to Temper 6, the latter material being the hardest and having a hardness of 65 to 70 R301 units in the as annealed condition (RBOT units are Rockwell hardness numbers using the superficial Rockwell tester with a 30 kg. load).

With known annealing treatments it is not possible to produce plate having the hardness required for Temper 6 from ordinary grade blackplate steels (that is low carbon steels containing less than 0.02% phosphorus), and it is at present produced from rephosphorised steels con taining approximately 0.1 to 0.14% phosphorus and up to 0.14% carbon. During annealing the phosphorous goes into solid solution in the ferrite and stiifens, i.e., hardens, the product, but with a significant reduction in ductility. The presence of phosphorus also lowers the corrosion resistance of the plate and gives non-uniform mechanical properties due to the occurrence of segregation of the phosphorus content in the ingot from which the strip is rolled.

-t is an obiect of the present invention to provide a process whereby annealed steel sheet and strip having a hardness of at least 65 RSOT units can be obtained from ordinary grade, that is non-rephosporised, blackplate steel.

Further objects and advantages of the present invention will appear from the description which follows.

According to the present invention, we provide a process for the production of annealed steel sheet and strip, which comprises heating cold worked steel sheet or strip which contains from 0.07 to 0.14% carbon md less than 0.02% phosphorus to a temperature of from 740 to 850 C., rapidly quenching the steel from this temperature to a temperature of from 150 to 250 C., and over-aging the steel within the latter temperature range whereby a stable product having a hardness of at least 65 R301" units is obtained.

The final hardness of the product after over-aging depends on the carbon content of the steel, the annealing temperature and, to a lesser extent, the quenching rate; thus in order to obtain the specified minimum hardness, it is necessary to employ a steel having a carbon content at the upper end of the specified range if an annealing temperature at the lower end of the specified range is used, or vice versa. More particularly, if an annealing temperature of from 740 to 770 C. (the "ice latter temperature being the Curie transition point for steel which is substantially independent of the carbon content) is used, the steel should contain from 0.12 to 0.14% carbon in order to obtain or exceed the specified minimum hardness. If annealing temperatures above the Curie point, that is from 770 to 850 C., are used, the specified minimum hardness can be obtained or exceeded with steels containing from 0.07 to 0.12% carbon.

If steels having a carbon content at the upper end of the specified range, say from 0.10 to 0.14%, are annealed at temperatures at the upper end of the specified range, say from 790 to 850 C., the final product will have a hardness greater than that of Temper 6 plate (which has a hardness of -70 R30T units), such a degree of hardness corresponding to that of so-called double-reduced tinplate. Double-reduced tinplate is produced by cold rolling tinplate after tinning so as to double its superficial area; owing to the increase in superficial area this product is much cheaper than conventional tinplate and although it has poorer properties than conventional tinplate, in respect, for example, of ductility, corrosion resistance, and uniformity of mechanical properties in the transverse and longitudinal directions with respect to the direction of rolling, the reduction in gauge is compensated by the considerable increase in hardness and stillness and the product can be used where the service conditions are not very severe. The present invention therefore provides a method of obtaining the necessary hardness in thin tinplate of this type (of 40 to 60 pounds substance, that is 0.0044 to 0.0066 inch thickness) by heat treatment rather than by cold rolling. Such a prodnot obtained by the present process has the advantages of greater ductility and less directionality over the product obtained by cold double reduction.

As mentioned above the hardness of the product after over-aging also depends to some extent on the quenching rate. Quenching may be effected into water or into a mol en metal bath, for example a bath consisting of a 50% mixture of lead and bismuth. The faster quenching rates obtainable by water quenching enable a somewhat harder product to be obtained than when a molten metal quenching medium is used. Thus products having satisfactory Temper 6 mechanical properties (hardness 65- R301 units) can be obtained with steels containing from 0.08 to 0.14% carbon by quenching from the Curie point, 770 (2., into water, but at the lower cooling rates associated with liquid metal quenching, the range of carbon contents which give Temper 6 hardness values using the same annealing temperature is narrowed to 0.12 to 0.14% carbon.

The steel sheet or strip is preferably heated rapidly to the annealin temperature, for example in less than 10 seconds, and is quenched immediately or after only a short holding time, i.e., less than 5 seconds. After quenching, the steel is over-aged at a temperature of from to 250 C. Over-aging is preferably carried out, in the case of strip treatment, by coiling the strip as it leaves the quenching bath and holding the coil in a thermally insulated container for the required time. The period requ red for over-aging depends on the overaging temperature, a longer time being required at lower temperatures and vice versa; in the middle of the temperature range specified, i.e., 200 C., over-aging is normally complete in /2 hour. The hardness of the final product is only slightly affected by the overaging temperature; lower over-aging temperatures within the above range giving slightly hart er products higher temperatures within the range. Over-aging can be carried out in air without any appreciable oxidation taking place at ten peratures below 200 C.; if over-aging is carried out at higher temperatures it is preferable to surround the steel with a non-oxidising atmosphere.

The process according to the invention is particularly suitable for the continuous high speed production of Temper 6 strip for the tinplate industry. Gwing to the very short processing times of the steps which take place (Steel A was over-aged at 230 C. for /2 hour, instead of at 200 C.)

The mechanical properties of these steels are given in Table 1 below, which alsoincludes the results obtained with control samples of a rephosphorised steel annealed commercially by a known process and then temper rolled and heat treated as described above:

Steels with the following Table 1 As temper rolled After strain aging 200 C min. As an- Thick- Steel healed, ncss,

R301 Erich- Erich- Jenkins ins.

R301? sen, R301 sen, Bend mm. mm.

67. 3 6S. 7 6. 73.1 5. 9/0 .0135 66.1 68. 3 7. 0 72. 2 6. 2 11/6 012 69. 2 70. 7 s. 4 74.1 5. 2 loll/7&5 .0115 Conuncrcial rcphosphorlscd 64. 7 67. 0 7. 55 70. 8 6. 6 12/8 .0135 7.10 71.2 7. 35 74. 0 5. 95 12/7 .0135

iii-line, namely up to 10 seconds for heating to the an- EXAMPLE 3 nealing temperature, not more than 5 seconds hold at this temperature, and about 5 seconds for quenching before the strip is coiled, the process can be operated continuously at high speed in plant which costs a fraction of that for the continuous high speed production of Temper 6 plate by present methods, and which occupies a fraction of the space required for the latter.

In order that the invention may be more fully understood, the following examples are given by way of illustration only:

EXAMPLE 1 EXAMPLE 2 Samples 4 /2 in. x 6 in. of cold worked steel sheet having different carbon contents were annealed under the conditions described in Example 1. After over-aging, the samples were temper rolled on a 6 in. mill with smooth rolls to 234-2344 extension and were then given a simulated lacquer stoving treatment by being maintained at Samples of cold worked steel sheet having different carbon contents were induction heated to the Curie point (770 C.) and quenched into a molten metal quenching bath consisting of 50% lead50% bismuth. Two heating cycles, namely heating to 770 C. in 5 seconds with 5 seconds hold (lstheating cycle) and to 770 C. in 3 seconds with 1 second hold (2nd heating cycle) were used; two bath temperatures, C. and 200 C. were used in conjunction with the 1st heating cycle and a bath temperature of 200 C. was used in conjunction with the 2nd heating cycle. Each sample was over-aged by being left in the bath for 30 minutes after quenching.

The hardness of the annealed samples (in R30T units) is set out in Table 2 below:

Table 2 2nd heating 1st heating cycle cycle Carbon content, percent Quench to Quench to Quench to 200 C.

EXAMPLE 4 stituents of the steels other than iron, are given in Table equivalent to a heating time of 15 seconds and a holding 3 below: time of 5 seconds).

Table 3 Annealing temperature, C 740 750 755 760 770 775 780 Steel analysis: 5

.050; .38 M11; .0223; .010? 6: 05 'II Z0- .070; .40 M11; .0338; .013P 7. 0

07. 5 .10C; .67 M11; .018S; .0121J 11 14C; .49 B111; .0205; .000P

.1266; .40 Mn; .0308; .011P 6.85

EV /A 13%[10 Annealing temperature, C 785 790 795 800 305 810 830 Steel analysis:

"ifi' .070; .40 Mn; .0335; .0131 633 15/9 73. 4 .100; .44 Mn; .0205; .0141 545 9/6 77.0 .100; .67 Mn; .018S; .0121 4 3 9/7 80. 9 .1260; .46 Mn; .0308; .011P 5.95 8/6 %Iorn.'lhe properties presented are in each case (reading downwards) Hardness (RT), Eriehsen (mm) and Jenkins Bend va ues.

It will be seen from these results that there is a con- Table4 sistent increase of hardness with annealing temperatures.

EXAMPLE 5 e0 filciii fii df s i f/5 0 72 353 Hardness 330T Small samples 2 inches x 1 inch or" cold worked steel Q5 Q5 7L6 sheet having different carbon contents were heated rapidly g in an electric induction heater to 770 C. with thermo- 14 3 5 I couple Wires spot welded to each sample and connected L g g to a .41 second f.s.d. recorder. The samples were heated 0.5 0.5 69:5 to and held at the annealing temperature for different g g 23:? times. They were quenched into a 50% lead50% bis- 3 10 69.8 math bath maintained at 200 C. and aged at the latter 3 5 gZ-i temperature for 30 minutes. 70 1 0.5 65.0

The hardness values of the samples after this treatment 3 g 22 1:; are given in Table 4 below, together with the comparative 3 10 e3. 9 values for similar samples which had been heated to the 1 15 2630 same annealing temperature (770 C.) in a salt bath by 1E immersion therein for 20 seconds (estimated as being 2 bath It will be seen from these results that the heating rate has little effect upon the hardness o tained with steel containing 0.10% carbon, while with the 0.14% carbon steel, fast heating rates give somewhat lower hardness and aging temperatures on hardness value" was also ascertained by heating samples of cold Worked steel sheet having diiferent carbon contents in an electric induction heater to 770 C., the heating time being respectively 1 values than salt bath heatin and with the 0.07% carbon 5 second, 5 seconds or seconds, quenching the samples steel, fast heating rates give somewhat higher values. into 50% lead-50% bismuth baths respectively mainw tained at 150 C. or 200 C. and over-win th samples CXAIVI"L11 6 n t t, at 150 C. for A hour or 200 C. for /2 hour. in order to demonstrate the eiiect of different quench- The hardness values obtained are set out in Table 6 ing rates. samples of 0.077% carbon cold Worked steel 10 below:

Table 6 1 sec. heating to 770 C. 5 sec. heating to 770 C. 10 sec. heating to 770 0.

Carbon content, Quench to Quench to Quench to Quench to Quench to Quench to Quench to ercent 150 0. 200 0. 150 C. a 0. 200 C. 150 200 C.

overage a1; ovcrage at overage at overage at overage at ovcrage at overnge at 1 200 0., 150 0., 150 0., 200 0., 150 2 /1 hr. /1 hr. 04 hr. 9 1 hr. n hr. }.1 111. la hr.

R301 RI R301 R301 R301 R301 R301 sheet were heated to 810 C. in a salt bath, quenched into The results obtained With the samples heated to the lead-50% bismuth baths maintained at 200 C., 250 C. and 350 C. reseectively and held in the baths for 30 minutes. In addition some of the samples were quenched into the baths at 350 C. and 250 C. and then hnrnediately transferred to other baths for holding at lower temperatures.

The results obtained by these treatments are given in Table 5 below:

annealing temperature in 5 seconds, show that the increase in hardness obtained by quenching to and over-aging at 150 C. is due to both the increase in quenching rate and the lowering of the over-aging temperature.

EXAMPLE 8 Samples of steel sheet 0.010 to 0.013 inch thick containing 0.0 to 0.125% carbon were cold rolled to a nominal Table 5 Holding temperature 350 0. 250 0. 200 C.

R301 Ehnrn.) J.B. R301 E'unm.) .T.B. R301 E(Inm.) .T.B.

Quench bath temperature . 18/15 61. 0 7.35 18/1/14 17% /16 62.6 6.7 Eli/14% 67.8 6.8 16/13 It will be seen that with steels having the low carbon content (0.077%) of the samples used in these treatments, the minimum haruuess for Temper 6 material R301 units) was only obtained with a quench bath ternperature and holding temperature of 200 C., higher quench bath and holding temperature (and therefore slower cooling rates from the annealing temperature) not enabling this hardness value to be obt ined.

thickness of 0.0055 inch. The cold reduced samples Were then heated to 810 C. by immersion for 4 seconds in a salt bath, quenched into a 50% lead-50% bismuth bath maintained at 150 C. and held at the latter temperature for 45 minutes. After this treatment, some of the samples were temper rolled to give a 2 /2% reduction and strain aged at 350 C. for 20 seconds.

The results obtained are set out in Table 7 below:

Table 7 An annealed (Qnenched As temper rolled and strain aged 1 The R301 values are converted R151 values (Wilson conversion chart).

results obtained also show that the eilect of different holding temperatures on the properties obtained after quenching to 35 C. and 250 C. is not very great, but

the effect of ouenchin tern erature itself is cons eerable,

es jecially over the range 250 to 200 C.

EXAMPLE 7 The hardnes of the treated samples containing 0.105% or more carbon is comparable to that of so-called doublereduced tinplate, that is tinplate of 40 to 60 pound su stance (corresponding to a thickness of 0.0044 to 0.0066 inch); but the ductility is greater and the directionality less, than that of double reduced tinplate (a typical example of the latter having Erichsen value of 3.0 mm. and

The effect or" dirlerent heat ng rates, quenching rates bend value of 25-1/2/7).

EXAMPLE 9 Samples (12 inches x inches) of cold reduced steel sheet having a carbon content of 0.14% were heated in a high frequency induction heater to 770 C., quenched in 3. A process for the production of annealed steel sheet and strip, which comprises the steps of heating cold worked steel sheet or strip which contains from 0.12 to 0.14% carbon and less than 0.02% phosphorous to a tem- Water and aged at C for 30 minutes The samples 5 perature of from 740 to 770 C., rapidly quenching the were then pivckled temper'mned to give 21/2 d steel from this temperature to a temperature of from 150 tion on a laboratory mill electrolytically tinned flow to 250 and than oilerdagmg the Steel W1 i the latter brightened SOved at C for 45 minutes i than temperature range until a stable product having a hard- L 1 K I stamped into standard beer can ends for double seaming mess of at F2 3 units. 18 Obtained, d 1 r and end Peaking tests 10 4. proce s ior the production of anneaie stee s ie et The hardness of the stovcd samples (75 R301 units) or strip 'whlch complies h Steps heating Cold was Sfighfly higher than the Specified limit for Temner 6 Worked steel sheet or strip which contains from 0.07 LO late, but no failures occurred during double seam form- 012% l g g' g if i il? 3; ing. Can peaking pressure values were, on average, 10% 33 il- I r t 2:: g i g a greater than the minimum requirement for beer cans and 8,, It g gig 0 to thp 12% greater than for cans made using a commercial rean en Y e w K phosphorised Ste e1 of the same gauge temper rolled 1m latter temperature range until a stable product having a 1 C7 3 J- der identical conditions. The average Erichsen and hardness of'at least 65 R30T i 15 Obtained Jenkins bend values for the finished Sam 16S wpre 6 1 5. A process for the production of annealed steel sneet and 9 1/2/5 1/2 res angel p 20 and strip, which comprises the steps of rapidly heating p cold worked steel sheet or strip which contains from 0.07 EXAMPLE 10 to 0.14% carbon and less than 0.02% phosphorus in less Cold reduced steel strip 5 inches Wide and containing than 10 W to tempjsrature of from 740 to 012% carbon 0.935% sulphur 001070 phosphorus and 850 C., holding the steel at said temperature for less than 043% manganesa was passed through a Continucus am 5 seconds, rapidly quenching the steel from said tempera nealing plant at approximately 150 feet/minute. In this l to a temperaiur? of from 150 to 250 and E plant the strip wa heated to an annealing temperature m the Steel Wlth1n.the tamperamre range a of in approximately 3 seconds and than quenched period of about minutes until a stable product having m 0 O in a molten 50% 16ad 5O% bismuth bath a hardness of at least 65 R30T units is obtained.

The strip was coiled as it came from the quench bath and 30 A accordmg to clalm Wherem 9 steel was maintained in the coil at 200 C. for 30 minutes. 15 quenched mm a 50% lead-50% blsmuth The strip was then temper rolled to 2 /2% extension A 9 Process, for the production of gnnealfid using shot blasted and smooth rolls and then electrolyticallsteel P, which omprises e steps of continuously 1y tinned (4 0L denosfl) and flow brightened on a cominw passing cold rolled steel strip which contains from 0.07 ous electrolytic tinning line. to 0.14% carbon and less than 0.02% phosphorus The tinned and flow brightening strip Was then subthrough a heating Zone Whfrein 931:1 ip is heated to jected to a lacquer stoving treatment before being tested. temperatur? of from p To 850 m 1353 than 10 The average results of these tests together with probable Sficonds, cofftlmwllsly Passlng the heated strip to a (111611011 values of a commercial rephosphorised Temper 6 plate h Whereln i 18 quenched a t mp rature of from are given in Table 8 below: 150 to 250 0., coiling the strip, and maintaining the Table 8 Sehopper values, kgJsq. mm. Average peaking pressure, percentage of min. require H J Plabte With grain Across grain mlelritlsbmorreited t)o -artlenl-tins su .sub once Samples ness Erielisen Bends stance a lbjbasis box Soft Beer can Yld.pt. Load at Y1 1.pt. Load at drink ends,

90 90 ends, percent percent Ernerimentaltinnlatm. 69-71 5.4-5.8 15/10 111416 45.5 57.5 03 103 102.5 Commercialtinplate... 69-70 5.9 11/6 114-118 42 54 45 07 i- No difficulties Were experienced during stamping and double seaming of the can ends and experimental tinplate compared favourably with rephosphorised steel. The only significant difierence in behaviour was found in the bend test results Where the experimental material exhibited les directionality and higher values than the commercial material.

I claim:

1. A process for the production of annealed steel sheet and strip, which comprises the steps of heating cold worked steel sheet or strip which contains from 0.07 to 0.14% carbon and less than 0.02% phosphorus to a temperature of from 740 to 850 C., rapidly quenching the steel irom this temperature to a temperature of from 150 to 250 C., and then over-aging the steel within the latter temperature range until a stable product having a hardness of at least R30T units is obtained.

2. A process according to claim 1, wherein the steel is quenched into a molten metal bath.

strip in the coil at a temperature of from to 250 C. for about 30 minutes until a stable product having a hardness of at least 65 R3011 units is obtained.

8. A process according to claim 7, wherein the quench bath consists of a 50% lead-50% bismuth mixture.

References Cited in the file of this patent UNITED STATES PATENTS Lose Oct. 30, 1945 OTHER REFERENCES 

1. A PROCESS FOR THE PRODUCTION OF ANNEALED STEEL SHEET AND STRIP, WHICH COMPRISES THE STEPS OF HEATING COLD WORKED STEEL SHEET OR STRIP WHICH CONTAINS FROM 0.07 TO 0.14% CARBON AND LESS THAN 0.02% PHOSPHORUS TO A TEMPERATURE OF FROM 740* TO 850*C., RAPIDLY QUENCHING THE STEEL FROM THIS TEMPERATURE TO A TEMPERATURE OF FROM 150* TO 250*C., AND THEN OVER-AGING THE STEEL WITHIN THE LATTER TEMPERATURE RANGE UNTIL A STABLE PRODUCT HAVING A HARDNESS OF AT LEAST 65 R30T UNITS IS OBTAINED. 